1. Field of the Invention
This invention relates to unsaturated polyester resin blends and to their use for the formulation of sheet molding compounds particularly useful for the manufacture of exterior automobile parts exhibiting high levels of toughness combined with Class A or finished sheet metal surface appearance.
2. The Prior Art
Unsaturated polyester resin compositions are extensively used in modern industry for the manufacture of relatively large durable articles of many types. Such compositions include an ethylenically unsaturated polyester resin which is the esterification reaction product of polyhydric alcohols and polycarboxylic compounds such as polycarboxylic acids and polycarboxylic anhydrides wherein at least a portion of the polycarboxylic compounds contain alpha, beta-ethylenic unsaturation.
These unsaturated polyester resins may be used in the formulation of molding compositions, referred to in the art as sheet molding compositions (SMC), which generally comprise an ethylenically unsaturated polyester resin as a major component dissolved in a suitable unsaturated cross-linking monomer containing terminal vinyl groups such as styrene and admixed with one or more thermoplastic resins, curing agents, polymerization inhibitors, catalysts, coloring agents, release agents, fibrous reinforcing materials, thickeners and fillers.
SMC systems can be molded into articles having different shapes and configurations by various molding processes such as compression molding, and injection molding under heat and pressure. One application of the SMC systems which is finding increasing acceptance and demand is in the manufacture of exterior automobile parts such as fenders, doors, tailgates and similar exterior car body parts.
The unsaturated polyester resins useful in preparing SMC systems for automotive applications are generally highly reactive unsaturated polyesters in which the acid component of the polyester consists of a high proportion of an alpha, beta-ethylenically unsaturated dicarboxylic acid. The SMC systems when compression molded at temperatures on the order of 200.degree.-330.degree. F. and pressures of about 700-1500 pounds per square inch (psi) can be formed into exterior automobile body parts which have a surface appearance resembling a finished sheet metal surface.
This finished sheet metal surface appearance is referred to in the art as a "Class A" surface which can be measured quantitatively as a surface number, by utilizing an instrument such as the Budd Surface Analyzer.TM. which is available from the Budd Company. Other devices can also be used such as the Loria.TM. (Ashland Co.) and Difracto Sight.TM. (Difracto Co.).
U.S. Pat. No. 4,540,741 to Corrado discloses alkyds of unsaturated polyesters obtained by reacting (a) 0.4-1.0 moles of an alpha, beta-ethylenically unsaturated acid such as maleic acid (b) 0-0.6 moles of another dicarboxylic acid such as adipic or dodecanedioic (c) 0.5-1.1 moles of a polymeric glycol such as 1,2 dipropylene glycol and (d) 0-0.5 moles of another glycol such as 1,2 propylene glycol. The unsaturated polyester when diluted with a vinyl monomer such as styrene can be used to prepare solid, expanded resinous materials suitable for an SMC system.
U.S. Pat. No. 4,292,218 to Corrado et al discloses polyester resins useful in the manufacture of buttons. The resins are prepared by the esterification of maleic acid optionally together with phthalic acid, adipic acid or dodecanedioic acid with one or more glycols and particularly dipropylene glycol together with 20-30% by weight of the total resin of a vinyl monomer such as styrene.
U.S. Pat. No. 4,276,388 to Iwami et al discloses a molding resin composition useful in the manufacture of car bodies which contains (a) 40-70 parts by weight of an unsaturated polyester formed from at least one alpha, betaunsaturated dibasic acid such as maleic acid or a mixture of saturated acids such as dodecanedioic acid, isophthalic acid and adipic acid reacted with a polyhydric alcohol such as propylene glycol to form an unsaturated polyester component and (b) 60-20 parts by weight of a dialkyl ester of an alkenyl succinic acid and 0-30 parts of an ethylenically unsaturated monomer such as styrene.
Although high reactivity unsaturated polyester resins based on high concentrations of alpha, beta-ethylenically unsaturated dicarboxylic acids are known to the art, these resins when considered for the formulation of SMC systems for exterior automobile body part applications lack the requisite "toughness", that is, the extent to which the body part can absorb energy necessary for applications involving doors, fenders and tailgates.
In the current manufacture of exterior automobile body parts, the industry demands that the body parts have a Class A surface combined with a high level of toughness. Despite their desirability, these combined properties have heretofore been difficult to obtain with conventional SMC systems formulated using highly reactive alpha, beta-ethylenically unsaturated acid based polyester resins. This is because cracking problems are encountered with high reactivity resins especially when the parts are hot, indicating lack of hot strength. The highly reactive resins can also have low impact strength and lack of flexibility.